Nuclear Ores and Its Life Cycle

¶ … Nuclear Fuel Cycle is a set of different processes that utilize nuclear materials and then returns them to their initial state, in a cyclical manner. It begins with the mining of naturally occurring nuclear materials from the environment, and ends with safe and proper disposal of nuclear waste products back to the environment. Production of energy from Uranium requires several unique processes.

One of the terms used in this production of nuclear energy is front end, referring to the entire set of processes involved in making nuclear energy from the uranium ore in the nuclear fuel cycle. The processes involved are: [1] mining, [2] crushing, [3] processing, [4] enrichment, and [5] the fabrication of fuel. After being used to produce energy, the nuclear material is now known as spent fuel. The spent fuel has to be converted in a reprocessing or storage facility if the company wants to recycle it.

Short- or long- term storage of the spent fuel, or the reprocessing of the same, are all known as the back end processes of the nuclear fuel cycle (IAEA, 2012). Uranium Ore Sources The cycle starts with mining or extraction of uranium (U) mineral. This mineral ore contains uranium as various complex oxides. These complex oxides are then reduced to the U30s oxide. This oxide is then taken through a conversion process to form uranium hexafluoride, UF6.

This is the form of uranium that is required and can undergo enrichment at isotope separation plants (Lamarsh and Baratta, 2001). Uranium deposits are found in different geological forms. The most important geological forms of Uranium are: the Proterozoic deposits which are mainly found in Australia and Canada; the roll fronts types in Mesozoic-Cenozoic deposits of the U.S.A. And Kazakhstan; and the Iron Oxide-Copper-Gold (IOCG) deposits whereby the uranium mineral is extracted as a by-product of the extraction of copper in Australia.

The mineralization process of Uranium that causes it to appear in different forms is determined by two of its properties: its high solubility in its hexavalent (U+6) state and its low solubility in a tetravalent U+4) state. The geochemical composition of Uranium is reflected in its widespread leaching by oxidized meteoric/formation waters and its precipitation in oxidized form, UO2, in redox reactions.

The loss of water from the soil through evaporation and plant transpiration in arid lands can result in the enrichment of Uranium near the surface in a calcrete form of deposits, such as in Australia and Namibia. Proterozoic Quartz-pebble conglomerates are also known to contain low-grade uraninite, such as in the Canadian and South African deposits. The present confirmed uranium deposits globally are enough to sustain present and predicted nuclear energy production demand for the next one hundred years.

Better and more advanced technologies in nuclear power production, such as fast neutron reactors, are expected to extend nuclear fuel lifetime to more than a thousand years (Lehmann, 2008). Mining Environmental Considerations The most important effect of any mining activity is its impact on the quality and flow of water in the area surrounding the mining project.

The main considerations are: [1] whether both the surface and groundwater resources will remain suitable for human consumption; and [2] whether the flow of surface waters will still be sufficient to support the indigenous aquatic and terrestrial life after the mining activity. Emissions into the atmosphere are part of every process of the mining cycle such as exploration, extraction, processing, construction, and other activities. Mining activities involve large amounts of raw materials and lots of waste materials which contain minute particles that can easily be blown by wind causing health hazards (ELAW, 2015).

Mining OSHA considerations Employees working in uranium mines are protected by regulations which place limits on their dust, noise, chemical and radiation dose exposures under the Mine Safety and Health Administration (MSHA), and the overall Occupational Safety and Health Administration (OSHA). Protection of uranium mine workers is specifically regulated by the Nuclear Regulatory Commission and is also subject to State legislation. States have full jurisdiction over mining activities on land that is not federally owned meaning that these activities are not regulated by federal agencies or policies.

Additionally, different mining companies also have different policies and procedures of limiting the exposure of their workers to different hazards. Eventually the legislations or policies that regulate the mining and processing of uranium are determined by both the type and location of mining activity. Thus, a mining facility depending on its own unique features can be governed by a combination of both federal and state and/or local employee protection and environmental regulations (Carlsen et al., 2013).

Enrichment Alternatives The milling or crushing of the Uranium oxides is not sufficient to make it a compatible nuclear fuel for use in a power plant, therefore additional conversions are required. Out of any amount of naturally occurring uranium, just a small percentage, 0.7%, can undergo fission, the process of producing power in a nuclear reactor. The only isotope of uranium capable of undergoing fission is the Uranium-235 (U-235). The other common isotope is the uranium-238 (U-238).

Many types of reactors require an increase in concentration of U-235, often by about 3.5-5%. This process is known as isotope separation, whereby an isotope is concentrated (enriched) relative to other isotopes. For enrichment to occur, the uranium needs to be in the gaseous form. This necessitates the conversion of the uranium oxide into uranium hexafluoride, which is gaseous at comparatively low temperatures. For reactors that do not use enriched uranium, the uranium oxide just has to be processed to uranium dioxide.

However, for the enrichment facility, uranium dioxide then has to be further processed into uranium hexafluoride. The biggest risk in this particular phase of the nuclear fuel cycle is in the utilization of Hydrogen Fluoride. Uranium hexafluoride once made is then pumped into huge metal cylinders, each with the ability to hold 14 tonnes. It is left for some time to solidify. These cylinders are then transported to enrichment facilities.

The separation process splits the uranium hexafluoride gas into two streams: one of which is being continuously enriched to the required concentration (low-enriched uranium), and the other which is increasingly being exhausted of the Uranium-235 (tails). The key enrichment process that is utilized in large scale production entails the use of centrifuges, composed of numerous fast-spinning tubes. The spinning action causes the two common uranium isotopes to separate based on their physical characteristics, particularly their one percent mass difference.

The final phase of enrichment is known as laser enrichment - this results in enriched Uranium hexafluoride which is then converted back to enriched uranium oxide. At this stage the nuclear fuel material can be regarded as fungible, however nuclear fuel preparation entails a very particular outline (WNA, 2014). Enrichment Processes Various enrichment methods have been demonstrated both historically and empirically; however, only two have been utilized on a large scale - the centrifugal process, and the diffusion process. Both of these processes entail the use of uranium hexafluoride as feed.

The particles of U-235 and UF6 are both one percent less heavy than the rest of the molecules and it is this particular physical difference that forms the foundation of both enrichment processes. Enrichment of isotopes is a physical method (WNA, 2015). The split streams of uranium hexafluoride produce low-enriched uranium and the "tails" (depleted uranium or DU). The former is used to produce nuclear fuel.

The amount of DU is vital because it indicates the total work done to produce a specific amount of enriched uranium out of a given amount of uranium feedstock. Raw materials have different concentrations of Uranium-235 based on their location of origin. Unused mineral ores have about 0.7% concentration of uranium, while recycled uranium has about 1%; the depleted uranium has nearly 0.25% concentration. The size of an enrichment facility is determined with respect to its Separative Work Units (SWU).

These are multi-part units that denote the energy produced, compared to the quantity of feedstock processed. Each SWU strictly measures in kilograms the amount of separation work done to enrich a particular quantity of uranium. The bigger unit-tonnes SWU-can also be utilized (World Nuclear Association, 2015). Below are four of the main processes used in enrichment. They include: Diffusion, centrifuge, electromagnetic, and laser processes. Each of the processes has different features.

The diffusion process is adaptable in response to fluctuations in demand and the cost of energy; however, it is highly energy intensive. The centrifuge process allows easy increase in capacity through modular expansion; it is however not highly adaptable. The laser process on the other hand has the ability to make the most use of depleted uranium by extracting the highest amount. It also has a capacity for modular expansion. 1. Centrifuge Process This particular enrichment process was first used in the 1940s.

It produces about 2 million SWU per year, making it economic and enabling the development of larger enrichment facilities. This process is highly energy efficient compared to the diffusion process; it requires only fifty to sixty kilowatt-hour per Separative Work Unit. Similar to the diffusion process, the centrifuge process utilizes Uranium Hexafluoride (UF6) gas as its feedstock, and is based on the 1% mass difference between the two uranium isotopes. UF6 is piped to vacuum cylinders, each of which is composed of a rotor 3-5 m tall and 0.2 m in diameter.

The spinning action of the rotors, at approximately 50,000-70,000 rpm, causes an increase in concentration of both isotopes, U-235 and U-238, at the center and the outer edges of the cylinders respectively. A thermal difference enables a counter-current flow that allows the enriched uranium to be taken out axially - the lighter isotopes on one end and the heavier ones on the other. The enriched uranium becomes the feedstock for the next phase, while the tail UF6 is transferred back to the previous phase.

In the end both the enriched and "tail" uranium are both taken out of the flow at the required levels. To ensure successful separation of U-235 and U-238, the rotors spin at very high rates with the outer wall of the mixture moving at rates of over 400M/s increasing the acceleration of gravity by almost a million. Even though a centrifuge has a smaller capacity than a diffusion stage, its enrichment capability is higher.

The stages of centrifuge process are composed of numerous centrifuges that are parallel to one another. The arrangement is in a cascade. This process has about ten to twenty stages unlike the diffusion one which may include up to a thousand or more stages (World Nuclear Association, 2015). 2. Laser Process This is a technologically advanced process that delivers more nuclear power and lesser DU at relatively lower capital input and lower feedstock, thus having huge economic benefits. There are two forms of laser inputs - molecular and atomic.

The atomic process is based on photo-ionization; this involves the use of a high-energy laser to ionize specific isotopes of the gaseous uranium. The laser light is at a specific frequency that ionizes Uranium 235 atom only and not a Uranium-238. The now positively charged Uranium-235 ions are attracted to a plate within the chamber that is negatively charged. This laser process can also be used to enrich plutonium isotopes. The principle behind this process is the use of laser radiation to break molecular bonds. This is known as photo-dissociation.

It involves the breaking of gaseous UF6 into solid positively charged UF5 ions. This then makes it possible to separate the UF5 ions from the UF6 gas which is composed of U-238 atoms. The fact that this process makes use of UF6 makes it easier to fit it into the traditional fuel cycle than the atomic one (World Nuclear Association, 2015). 3. Diffusion Process Commercial scale uranium enrichment was first done via the diffusion process. It was first done in the U.S.A.

And it was later carried out in France, Russia, the United Kingdom (UK), and China. It is a highly energy intensive process consuming about 2500 kilowatt-hour per Separative Work Unit. The diffusion process accounts for about a quarter of the total world enrichment capacity. Gaseous diffusion facilities are reliable and long lasting. The process involves pumping the UF6 gas under high pressure through a sequence of porous membranes. The Uranium-235 molecules, being 1% lighter than the Uranium-238 molecules, travel faster and have a higher probability of moving through the pores.

Therefore the UF6 gas that comes out of the membrane is more enriched than that which did not pass through (World Nuclear Association, 2015). This process is repeated numerous times through a sequence of diffusion phases known as a cascade. Each phase is composed of a diffuser, a compressor, and lastly a heat exchanger to take away the heat due to compression. The enriched UF6 is drawn out through one end and the depleted uranium through another. There are about 1400 phases in this process, resulting in U-235 that is 3-4% enriched.

The diffusion facilities basically have a lower capacity of enrichment through one phase, thus the higher number of phases is required. However, they have the capacity to handle large amounts of gases (World Nuclear Association, 2015). 4. Electromagnetic process This approach was one of the initial attempts at enrichment of isotopes. The earliest one was process was Electromagnetic Isotope Separation (EMIS) which utilized calutrons. A calutron is defined as being a 'mass spectrometer specifically used in separation of U. isotopes'.

Use of the EMIS method was part of the famous Manhattan Project, whose purpose was to develop the well enriched uranium that was later utilized to make the Hiroshima Atomic bomb. This process uses the principles of the mass spectrometer at a commercial scale. The ions of Uranium isotopes U-235 and U-238 are separated based upon the different arcs they describe when traveling through the magnetic field due to their different atomic radii. The electromagnetic process is generally ten times more energy intensive than the diffusion one (World Nuclear Association, 2015).

Enrichment Facilities The International Atomic Energy Agency (IAEA), and in conjunction the Global Nuclear Energy Partnership (GNEP), proposed the establishment of international nuclear enrichment facilities. The drive behind this proposal was to bring all new enrichment capabilities under international control as a step to prevent proliferation of nuclear weapons among other things; it was also an attempt to establish more unified and appropriate disposal of nuclear waste. The international control would enable more scrutiny than is the case in restricted government controlled facilities.

The first of these international enrichment facilities was the International Uranium Enrichment Centre (IUEC) located at Angarsk in Siberia and co-owned by Ukraine, Kazakhstan, and Armenia (World Nuclear Association, 2015). The objective of setting up IUEC was to have a constant supply of enriched uranium for use in nuclear power plants in the partner states, while restricting access to enrichment technology. Russia is the majority shareholder in IUEC.

The facility was also recently added to the family of Russian nuclear facilities enabling scrutiny by the IAEA to ensure that safeguards are met. Eurodif is one of the other international enrichment facilities. It is located in France, which is also the majority shareholder at 60%. The plant is co-owned by Spain, Italy, Iran, and Belgium and is regulated by IAEA safeguards while restricting access to the enrichment technology. There is also a provision allowing partner states (except Iran) to share the enriched uranium (World Nuclear Association, 2015).

The French Atomic Energy Commission suggested that the replacement of Eurodif, the Georges Besse II facility, should also be open to co-ownership just like its predecessor. This has enabled the sale of shares by Areva to a Japanese consortium, GDF Suez, and the Korea Hydro and Nuclear Power (KHNP). Urenco is one of the other international enrichment facilities under the co-ownership of Germany, Britain, and Holland. However in Urenco, the partner states do not have access to enrichment technology.

A new Areva plant to be set up in the U.S.A. will be strictly under the control of the company with no co-ownership options. It will basically be a French firm on U.S. soil. The other main enrichment facility in western countries is USEC located in the United States (World Nuclear Association, 2015). Fuel Pellet Manufacture, including materials considerations and Fuel Pellet specifications Biomass pellets are known to produce high quality nuclear fuel compared to the conventional feedstock.

The pellets produce more energy per unit quantity and can be handled easily. The pellets can also easily be utilized in automated input systems. As well, the biomass pellets are also more sustainable and less harmful to the environment making them very valuable to use. The conventional fuel pellet is in the form of a cylinder with a radius of about 4mm and a height of less than 38mm. At times larger pellets are made. These are known as "briquettes" and are usually larger than 12 mm in radius.

A superior quality fuel pellet is firm, dry and durable; it also contains low quantities of ash post-combustion. As per the Pellet Fuels Institute (PFI), superior quality pellets are those which contain less than 1% of ash, while standard pellets contain up to 2% of ash. All pellets must have no more than 300 ppm of chloride levels, while the dust should also be less than 0.5% chloride. Additionally grasses and plant debris cause the production of ash which causes clumps at extremely high temperatures.

Due to this pellet, stoves used to burn wood are unsuitable for burning other forms of pellets apart from wood. Rather, biomass pellet-specific stoves that are built particularly for the biomass pellets should be utilized (CAS, 2009). Fuel assembly manufacture The end stage of preparing nuclear fuel is known as fuel fabrication. Different nuclear fuel fabrication facilities produce different types of nuclear fuels to be used in particular reactors. Nuclear facilities and fuel fabricators have worked together to bring about efficiency in the fuel fabrication.

Nuclear reactors produce energy by using enriched nuclear material. The process known as the fission process produces very high amounts of energy; therefore it must be carried out in a strong solid compartment specifically built to handle the extreme temperatures and the high radiation environment. These compartments known as fuel structures must be durable and properly maintained to ensure their integrity is not compromised; thus,.